The two most common conventional non-volatile data storage devices are disk drives and solid-state random access memories (RAM). Disk drives are capable of inexpensively storing large amounts of data, i.e., greater than 100 GB. However, disk drives are inherently unreliable. A hard drive comprises a fixed read/write head and a moving medium upon which data is written. Devices with moving parts tend to wear out and fail. Solid-state random access memories currently store data on the order of 1 GB (gigabyte) per device, and are relatively expensive, per storage unit, compared to a disk drive.
The most common type of solid-state RAM is flash memory. Flash memory relies on a thin layer of polysilicon that is disposed in oxide below a transistor's on-off control gate. This layer of polysilicon is a floating gate, isolated by the silicon from the control gate and the transistor channel. Flash memory is relatively slow, with reading and writing times on the order of a microsecond. In addition, flash memory cells can begin to lose data after less than a million write cycles. While this may be adequate for some applications, flash memory cells may begin to fail rapidly if used constantly to write new data, such as in a computer's main memory. Further, the access time for flash memory is much too long for computer applications.
Another form of RAM is the ferroelectric RAM, or FRAM. FRAM stores data based on the direction that ferroelectric domains point. FRAM has access times much faster than Flash memory and consumes less energy than standard dynamic random access memory (DRAM). However, commercially available memory capacities are currently low, on the order of 0.25 MB (megabyte). In addition, memory storage in a FRAM relies on physically moving atoms, leading to eventual degradation of the medium and failure of the memory.
Yet another form of RAM is the Ovonic Unified Memory (OUM) that utilizes a material that alternates between crystalline and amorphous phases to store data. The material used in this application is a chalcogenide alloy. After the chalcogenide alloy experiences a heating and cooling cycle, it can be programmed to accept one of two stable phases: polycrystalline or amorphous. The differences in the respective resistances of the two phases allow the chalcogenide alloy to be used as memory storage. Data access time is on the order of 50 ns. However, the size of these memories is still small, on the order of 4 MB currently. In addition, OUM relies on physically changing a material from crystalline to amorphous, likely causing the material to eventually degrade and fail.
Semiconductor magnetoresistive RAM (MRAM) encodes data bits in a ferromagnetic material by utilizing the direction of the material's magnetic moment. Atoms in ferromagnetic materials respond to external magnetic fields, aligning their magnetic moments to the direction of the applied magnetic field. When the field is removed, the atoms' magnetic moments still remain aligned in the induced direction. A field applied in the opposite direction causes the atoms to realign themselves with the new direction. Typically, the magnetic moments of the atoms within a volume of the ferromagnetic material are aligned parallel to one another by a magnetic exchange interaction. These atoms then respond together, largely as one macro-magnetic moment, or magnetic domain, to the external magnetic field.
One approach to MRAM uses a magnetic tunneling junction as the memory cell. The magnetic tunneling junction comprises two layers of ferromagnetic material separated by a thin insulating material. The direction of the magnetic domains is fixed in one layer. In the second layer, the domain direction is allowed to move in response to an applied field. Consequently, the direction of the domains in the second layer can either be parallel or opposite to the first layer, allowing the storage of data in the form of ones and zeros. However, currently available MRAM can only store up to 1 Mb (megabit), much less than needed for most memory applications. Larger memories are currently in development. In addition, each MRAM memory cell stores only one bit of data, thereby limiting the maximum possible memory capacity of such devices.
A magnetic shift register replaces many conventional memory devices including but not limited to magnetic recording hard disk drives, and many solid-state memories such as DRAM, SRAM, FeRAM, and MRAM. The magnetic shift register provides capacious amounts of storage comparable to those provided in conventional memory devices but without any moving parts and at a cost comparable to hard disk drives.
Briefly, the magnetic shift register memory device uses the inherent, natural properties of the domain walls in ferromagnetic materials to store data. The magnetic shift register memory device utilizes one read/write device to access numerous bits, on the order of 100 bits of data or more. Consequently, a small number of logic elements can access hundreds of bits of data.
The magnetic shift register memory device uses spin-based electronics to write and read data in ferromagnetic material so that the physical nature of the material in the magnetic shift register is unchanged. A shiftable magnetic shift register comprises a data track formed of a fine wire or strip of material made of ferromagnetic material. The wire can be comprised of a physically uniform, magnetically homogeneous ferromagnetic material or layers of different ferromagnetic materials. Information is stored as direction of magnetic moment within the domains in the track. The wire can be magnetized in small sections in one direction or another.
An electric current is applied to the track to move the magnetic domains along the track in the direction of the electric current, past reading or writing elements or devices. In a magnetic material with domain walls, current passed across the domain wall moves the domain wall in the direction of the current flow. As the current passes through a domain, it becomes “spin polarized”. When this spin-polarized current passes into the next domain across a domain wall, it develops a spin torque. This spin torque moves the domain wall. Domain wall velocities can be very high, on the order of 100 to 500 m/sec.
In summary, current passed through the track (having a series of magnetic domains with alternating directions) can move these domains past the reading and writing elements. The reading device can then read the direction of the magnetic moments. The writing device can change the direction of the magnetic moments, thus writing information to the track.
What is needed is an improved method for fabricating the magnetic data tracks needed to build a magnetic shift register memory device.